Organic two-dimensional crystals (O2DCs) are promising metal-free platforms for photocatalytic water splitting (PWS), owing to their tunable optoelectronic properties, extended π-conjugation, and structural porosity. However, the independent role of network topology in governing the photocatalytic performance remains unclear. Here, we employ density functional theory and nonadiabatic molecular dynamics to investigate eight topological isomers of O2DCs featuring single-pore rhombic (rmb) and dual-pore kagome (kgm) lattices, constructed from experimentally accessible building blocks, tetrathieno-naphthalene (TTN) and tetrathienylethene (TTE) linked via alkene or imine (Bz-I) moieties. The core-linker chemistry predominantly sets optoelectronic properties of O2DCs, including bandgaps (1.03–1.86 eV), optical absorption range, and HER/OER thermodynamics, whereas network topology governs the carrier dynamics in PWS. In particular, kgm crystals exhibit pronounced asymmetry in electron-hole transport, spatially separated charge carriers, and substantially prolonged electron-hole recombination lifetimes compared to their rhombic counterparts, where unidirectional anisotropic transport leads to rapid recombination. Among all candidates, kgm-TTN-Bz-I O2DC shows the most favorable band alignment and extended carrier lifetimes (104.91 ps), making it a promising metal-free light-harvesting/charge-separation scaffold for overall PWS with the aid of an OER co-catalyst and/or applied bias. This work establishes topology engineering as a practical design strategy for developing efficient and sustainable metal-free photocatalysts.